- Mechanistic Studies of Gas-Phase Negative Ion Unimolecular Decompositions. Alkoxide Anions
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The unimolecular decompositions of 15 gas-phase alkoxide negative ions have been studied by infrared multiple photon photochemical activation in an ion cyclotron resonance spectrometer.Upon pulsed CO2 laser irradiation, alkoxide anions undergo elimination of neutral molecules (e.g., alkanes RH) to yield enolate anions.The observed reactivity patterns and kinetic isotope effects further establish a stepwise decomposition mechanism involving initial heterolytic cleavage to an intermediate anion-ketone complex followed by proton transfer to give the ultimate products.A relative order of leaving group propensities CF3 > Ph > H > t-Bu > Me > i-Pr > Et was observed.The apparent anomalous reactivity order for the alkyl groups can be rationalized by invoking a change in mechanism to one involving an intermediate in which an electron is not bound specifically by the eliminated alkyl group for R + t-Bu, i-Pr, and Et: either a radical-ketone radical anion complex produced by homolytic cleavage or an anionic cluster.This order also leads to the conclusion that methane elimination from alkoxide anions proceeds via the pathway involving heterolytic cleavage.The results of this study have implications for bimolecular ion-molecule reaction dynamics, since the photochemically generated intermediates are also intermediates in bimolecular proton transfer reactions.
- Tumas, William,Foster, Robert F.,Brauman, John I.
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p. 2714 - 2722
(2007/10/02)
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- A Stepwise Mechanism for Gas-Phase Unimolecular Ion Decompositions. Isotope Effects in the Fragmentation of tert-Butoxide Anion
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Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an cyclotron resonance (ICR) spectrometer has been used to the mechanism of a gas-phase negative ion unimolecular decomposition.Upon irradiation with a CO2 laser (both high-power pulsed and low-power continous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane.The mechanism of this formal 1,2-elimination reaction was probed by measuring hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions.Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed.These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex.The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory.The implications of this study for several previously reported unimolecular ion decompositions are also discussed.
- Tumas, William,Foster, Robert F.,Pellerite, Mark J.,Brauman, John I.
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p. 961 - 970
(2007/10/02)
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- Generation, Thermodynamics, and Chemistry of the Diphenylcarbene Anion Radical (Ph2C.-)
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Dissociative electron attachment with Ph2C=N produced Ph2C.- (m/z 166).The reactions of Ph2C.- with potential proton donors of known gas-phase acidity were used to bracket PA(Ph2C.-) = 380 +/- 2 kcal mol-1 from which ΔHf0(Ph2C.-) = 81.8 +/- 2 kcal mol-1 was calculated.The reactions of Ph2C.- with CH3OH and C2H5OH proceeded with major and minor amounts, respectively, of a H2.+-transfer channel, forming Ph2CH2, RCHO, and an electron.The kinetic nucleophilicity of Ph2C.- in SN2 displacement reactions with CH3X and C2H5X molecules was shown to be medium, which requires a significant intrinsic barrier in these reaction.The reactions of Ph2C.- with various aldehydes, ketones, and esters were fast and established two principal product-forming channels: (1) H+ transfer if the neutral reactant contains activated C-H bonds and (2) carbonyl addition followed by radical β-fragmentation of one of the groups originally attached to the carbonyl carbon.The order for the ease of radical β-fragmentation in the tetrahedral intermediates was RO > alkyl >> H, and CO2CH3 > CH3.Since the reactions of Ph2C.- with the simple esters HCO2CH3 and CH3CO2CH3 were fast, it should now be possible to examine the reactions of carbonyl-containing organic molecules, which are expected to react slower than these esters and obtain their relative reactivities.
- McDonald, Richard N.,Gung, Wei Yi
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p. 7328 - 7334
(2007/10/02)
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- Gas-Phase Nucleophilic Reactivities of Phenylnitrene (PhN-*) and Sulfur Anion Radicals (S-/.) at sp3 and Carbonyl Carbon
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The reactions of PhN-/. with a series of carbonyl-containing molecules (aldehydes, ketones, and esters) were shown to proceed via an addition/fragmentation mechanism, PhN-* + R2C=O -> -)R2> -> PhN=C(O-)R + *R, producing various acyl anilide anion products.In several cases, the tetrahedral intermediate anion radicals were observed as minor ions.The intrinsic reactivity of the carbonyl-containing molecules was aldehydes > ketones > esters, where similar R groups were involved.The overall exothermicities of these reactions did not appear to play the major role in determining the relative rates (krelC=O) for these reactions.From the reaction of PhN-* with cyclobutanone, a new type of anion radical, PhN=C(O-)CH2* (m/z 133) (+ C2H4) was produced; the loss of C2H4 was considered due to the ring strain in the ketone.With cyclopentanone, cyclohexanone, and cycloheptanone, the anion radicals PhN=C(O-)(CH2)n* (n = 4-6) were the exclusive product ions.PhN-* was shown to be a poor nucleophile in SN2 displacement reactions with CH3X molecules (X = Cl, Br, O2CCF3).S-* was shown to exhibit modest SN2 nucleophilicity with CH3Cl and CH3Br.The reactions of S-* with CF3CO2R proceed via both SN2 displacement and carbonyl addition/fragmentation mechanisms: with R = CH3, the anion products were 65percent CF3CO2- and 35percent CF3COS-; from R = C2H5, the product ions were 4percent CF3CO2- and 96percent CF3COS-.These data yield the ratio kCH3/kC2H5 = 16 for SN2 displacement by S-* at these alkyl groups.The reactions of PhN-* with CO2, COS, CS2, and O2 are also reported.The reaction of PhN-* with CS2 to produce S-* as a major channel was used as the source of this atomic anion radical.In several reactions occuring at nearly the collison limit, selectivity was observed for (a) which of two reaction centers were attacked to give products and (b) which of two mechanisms would be dominant in the overall reaction.
- McDonald, Richard N.,Chowdhury, A. Kesem
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p. 198 - 207
(2007/10/02)
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- INFRARED STUDY OF THE REACTIVITY OF ACETONE AND HEXACHLOROACETONE ADSORBED ON HAEMATITE
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Infrared spectra of acetone adsorbed at beam temperature on α-Fe2O3 show that acetone can chemisorb on Lewis-acid sites and gives, at least partially, enolate anions; these, by aldolic condensation with molecules from the gas phase, produce a chemisorbed form of mesityl oxide.At 523 K acetate ions are formed.Two different forms of trichloroacetate ions are formed on the surface at beam temperature by the adsorption of hexachloroacetone and trichloroacetic acid.This behaviour indicates the presence of pairs of acid-base sites on the surface of haematite, and also shows that its surface hydroxy groups have a lower degree of nucleophilic character with respect to those of other oxides such as aluminas, SnO2 and alkaline-earth oxides.
- Busca, Guido,Lorenzelli, Vincenzo
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p. 2911 - 2920
(2007/10/02)
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